1. Field of the Invention
This invention relates to a process of preparing a vinyl chloride polymer, and more particularly to a process of preparing vinyl chloride polymer, that can prepare a vinyl chloride polymer of high quality with a high productivity.
2. Description of the Prior Art
In preparing vinyl chloride polymers, it may sometimes occur that, after a polymerization initiator has been fed into a polymerization vessel through a charging passage comprising a charging pipe, etc., the polymerization initiator sticks on the inner wall of the charging passage, particularly at a charge port positioned at the end of the charging passage to remain there. The polymerization initiator remaining like this reacts with a monomer during the polymerization, resulting in formation of polymer scale. Since the polymer scale may grow to clog the charging passage, particularly at the charge port, the grown polymer scale must be removed.
As methods of removing the polymerization initiator having stuck and remaining in the charging passage and of preventing formation of the polymer scale, proposed are a method in which the charging passage is washed with an organic solvent, a method in which the charge port is closed to block the invasion of the monomer from the inside of the polymerization vessel, etc.
However, in the former method, in which the charging passage is washed with an organic solvent, a large quantity of the organic solvent is required for completely removing the polymerization initiator having stuck and remaining therein, so that the solvent remains in a manufactured polymer to cause a lowering of quality, and generation of an ill smell when the polymer is molded or formed. Hence, there is a limit on the amount of the organic solvent to be used, and thus the washing can not be thoroughly performed. As the results, the polymerization initiator remains more or less, bringing about the problem that the polymer scale are still formed if the polymerization is carried out for a long period of time. In the latter method, in which the charge port is closed, there has been the problem that the monomer invades into the charging passage from a gap in the closed charge port resulting in the formation of polymer scale.
There has been also proposed a method in which the polymerization initiator is charged as an aqueous emulsion in order to reduce the amount of an organic solvent to be used. This method can make it avoidable that the organic solvent remains in a manufactured polymer, but yet it is difficult to prevent the polymerization initiator from sticking and remaining in the charging passage, and to prevent the scale formation in the charging passage.
Yonezawa et al. discloses in the U.S. Pat. No. 4,320,215, a method of polymerizing vinyl chloride monomer, wherein a coating agent capable of inhibiting radical polymerization is coated on the inner surfaces of the polymerization vessel and associated equipment used in the polymerization of vinyl chloride monomer, to prevent polymer scale formation and adhesion. The associated equipment is exemplified by propellers and agitators. (See column 4, lines 20-21).
Kleine et al. discloses in the U.S. Pat. No. 4,661,569 discloses a process of polymerizing ethylenically unsaturated monomers including vinyl chloride in the presence of a free radical-forming initiator in a reactor, wherein the surfaces of the reaction container wall and other equipment including pipelines are coated with a particular compound.
According to the method or the process disclosed in the above patents, the coating formed on the surfaces of a polymerization vessel, etc. needs to be insoluble in vinyl chloride monomer and aqueous medium (See column 2, lines 45-49 in the U.S. Pat. No. 4,320,215; and column 2, lines 45-51 and column 8, lines 16-21 in the U.S. Pat. No. 4,661,569), because it is required to enhance the durability of the coating and its scale preventing action, and it is also required to prevent product polymer or waste water from being contaminated with the material of the coating. Therefore, the method or the process is suitable for preventing polymer scale formation on the inner wall of a polymerization vessel and on the surfaces of equipment such as propellers, agitators and baffles used in the polymerization vessel. However, the method or process is not effective in preventing scale formation in passages for charging polymerization initiators for the following reason. After a polymerization initiator solution is charged in a polymerization through a charging passage, part of the solution containing the polymerization initiator remains in the passage and stays in various depressions or just above valves provided in vertical portions of the passage. Since the coating dissolves in the standing solution with difficulty, the standing solution containing the polymerization initiator is contact with the coating formed on the inner surfaces of the passage only at the surface of the coating. In other words, the mass of the standing solution are not in contact with the coating; most of the polymerization initiator remains active. Thus, if vinyl chloride monomer gas leaks via a valve from the inside of a polymerization vessel into the passage, the monomer is polymerized by the polymerization initiator contained in the standing solution.